1. Technical Field
The invention relates to vehicle tires and particularly to a run flat pneumatic tire containing a thin annular band which acts as a structural compression member when the tire is in the unpressurized or uninflated state to enable the tire to run in this condition. More particularly, the invention relates to a composite band element comprising a plurality of wound layers of strip material contained in a resin matrix which has a varying radial thickness to enhance the tire ride quality and band element durability for both inflated and uninflated conditions.
2. Background Information
Various tire constructions have been devised over the years which enable a tire to run in an under-inflated or non-inflated condition, such as after receiving a puncture and loss of pressurized air for extended periods of time and at relatively high speeds. This enables the operator to safely drive the vehicle to an appropriate location for repair or replacement of the punctured tire. Certain of these safety tires, referred to as xe2x80x9crun flat tiresxe2x80x9d, have been successful for certain applications and certain types of tire constructions. Most of these run flat tires achieve their run flat capability, by the placement of reinforcing layers or members of relatively stiff elastomeric material in the sidewalls of the tire which enable the tire to support the vehicle weight even with the complete loss of internal air pressure. Some examples of such prior art run flat tire constructions which use such sidewall inserts are shown in U.S. Pat. Nos. 4,287,924; 4,365,659; 4,917,164; and 4,929,684.
In addition to these prior art run flat tires having sidewall reinforcements, various run flat tire constructions have been developed which utilize a thin annular band element which extends circumferentially throughout the tire beneath the tread area in the crown portion of the tire. Some examples of such banded run flat tires are shown in U.S. Pat. Nos. 4,673,014; 4,794,966; and 4,456,048.
Banded tire elements have been fabricated with materials made of steel, aluminum, titanium, and epoxy and thermoplastic composites with glass, KEVLAR (aromatic polyammide) and graphite fiber reinforcement. A possible failure mode with these lightweight, laminated band constructions is interlaminar shear which usually occurs along the band""s primary bending or neutral axis. This is a fatigue failure and is directly related to the spectrum of cyclic operating stress. As in all fatigue failures, the lower the stress, the longer the operating life.
Various band element configurations have been developed in an attempt to eliminate or materially reduce interlaminar shear stress. One example is shown in U.S. Pat. No. 6,112,791, wherein the band element has tapered ends for improving interlaminar shear strength at the ends of the band element. Other prior art band elements have chamfered ends to reduce the interlaminar shear stress at the band ends. However, all of these prior art band elements have a generally constant radial thickness in an axial direction and a rectangular cross section throughout the majority of the axial length of the band element. In these band elements having the constant radial thickness, initiation or onset of interlaminar shear failure is located near the axial midpoint of the band element near the neutral axis.
The inflated ride quality, comfort and harshness of a banded run flat tire is effectively controlled by the circumferential rigidity and bending stiffness of the band element within the central footprint region. The circumferential rigidity necessary for the band element to adequately carry tire loads in both the inflated and uninflated conditions, is largely determined by the bending stiffness of the band element ahead of and behind the central footprint region. Likewise, it is desirable that the band element has increased interlaminar shear strength and increased circumferential strength or stiffness in order to reduce failure of the band.
It has also been learned that band elements having dual stiffness characteristics have been effective in achieving a better run flat tire. A dual stiffness band has variable axial bending stiffness with a lower axial bending stiffness in the central footprint region which improves inflated ride quality, comfort and harshness, yet provides a higher axial bending stiffness ahead of and behind the central footprint region in order to provide the necessary band durability and load carrying capacity. Several band element configurations have been disclosed in the prior art which provide for a dual stiffness result. One such band configuration for providing the dual modulus effect is to use a band element having an anticlastic shape such as shown in U.S. Pat. No. 4,456,084.
In summary, the present invention provides a run flat tire and in particular a band element therefor, which has increased interlaminar shear strength when the tire is operating both in the inflated and deflated conditions by forming the band of a composite material with a varying radial thickness from the axial midpoint towards the axial ends.
Another feature of the invention is to provide a band element for a run flat pneumatic tire which is formed of a composite material from various types of fibers, preferably graphite, fiberglass or a combination of both, which fibers are in strip materials and are encased in a resin matrix and provided with varying radial thickness to provide the desired stiffness band.
A further aspect of the invention is to provide a band with optimized weight and stiffness by providing the desired radial thickness at the axial midpoint of the band while reducing the radial thickness toward the axial ends thereby decreasing the band weight without sacrificing the interlaminar shear strength and durability.
Still another aspect of the invention is providing a band element wherein the radial thickness varies from the midpoint to the axial ends with the band cross section being symmetrical or non-symmetrical with respect to the neutral axis of the band element.
Another aspect of the invention is to provide a band element wherein the ends have a radial thickness between twenty percent and eighty percent of the midpoint thickness of the band element.
Another feature of the invention is to provide a band element formed of wound layers of resin impregnated materials which form a plurality of layers providing curved axially extending surfaces between the layers within the band, which surfaces cross the neutral axis of the band due to the curvature of the band, to further increase interlaminar shear strength.
Still another aspect of the invention is providing the band with an anticlastic configuration to provide the band element with a dual stiffness. The term xe2x80x9canticlasticxe2x80x9d is defined as a double curved shape where the surface curve is concave in one direction and convex in another. This dual stiffness provides an improved ride quality and comfort by having a lower axial bending stiffness in the central footprint region of the tire and provides the required durability and strength by providing a higher axial bending stiffness in the band ahead of and behind the central footprint region as the tire rotates.
Furthermore, the anticlastic band element of the present invention has a concavity within the range of {fraction (1/10)} to twice the radial thickness of the outer ends of the band element, and preferably has a concavity equal the thickness of the outer ends of the band element.
A further aspect of the invention is to enable the band element to be fabricated by usual methods of fabrication available for prior composite band elements, such as homogenous filament winding, nonhomogenous filament winding, multi-layer tape composite winding, winding with preimpregnated materials, winding with wet woven materials, winding with mats, winding with resin transfer molding processes, winding with wet or preimpregnated woven performs, and combinations of these prior known composite band forming techniques and methods of fabrication.
Another feature of the invention is to provide a band element formed of composite materials which allows tailoring of both stresses and elastic deflection properties by controlling orientation of the fibers and selection of fiber modulii in the strip material.